The present invention relates to a ball joint to be used, for example, in a steering mechanism or in a suspension mechanism of a vehicle.
A conventional ball joint typically includes a housing having an inner chamber into which a hard plastic bearing seat is fitted. A ball head of a ball stud is rotatably engaged in the bearing seat. A projecting stud section, which is integrally connected to the ball head, extends through an opening of the inner chamber. A plug, fitted into the opening, holds the bearing seat and the ball head in the inner chamber of the housing.
The above prior-art ball joint presents a problem in that it is difficult to absorb dimensional tolerances of the parts that comprise the housing, the bearing seat, the ball stud and the plug. As a result, it is difficult to stabilize the characteristics of a ball joint when it is assembled with a tie-rod or other mechanical part. The difficulty in controlling tolerances carries with it the danger that the bearing seat may wear prematurely and that rattling may occur with use due to permanent deformation of the bearing seat caused by high loads.
One approach to solving the above problems is disclosed in Japanese Patent Publication No. 44620-1978. A ball joint according to the above patent includes a ball head of a ball stud fitting into a bearing seat, which has both resiliency and rigidity. The slide surfaces of the seat have a radius of curvature larger than the diameter of the ball head. The ball head is slidably supported in the bearing seat by two annular slide surfaces along the inner surface of the bearing seat. The bearing seat is then slipped into an inner chamber of the ball joint's metallic housing. Both ends of the inner chamber are open. A sealing member is then pressed against an annular ridge at the opening through which the ball stud protrudes. A dome-shaped section of the bearing seat and of the housing is formed to substantially envelope the ball head. An elastic deformation occurs in the bearing seat as the dome-shaped section of the ball joint is formed.
The elastic restoring forces arising during the formation of the dome shaped section enable the ball joint to compensate for dimensional tolerances of the principal components, thereby ensuring a more uniform working torque and generally smoother operation. The bending deformation to form the dome-shaped section also relieves loads applied to the ball stud, thus enabling the ball stud to withstand higher loads.
A ball joint described in the above Japanese Patent Publication No 44620-1978 has a problem, however, when a large load or sustained heavy load is applied to its ball stud. The load bearing surfaces of the bearing seat are exposed to the increased load, which can cause permanent deformation and other damage to the bearing seat.
Another approach to solving the above problems is disclosed in U.S. Pat. No. 5,188,477. A ball joint according to the above patent includes a housing having an inner chamber with a hard plastic bearing seat and with an elastic member interposed between the bearing seat and a bottom of the inner chamber. The ball joint resiliently positions a section of its hard plastic bearing seat a short distance from a load-receiving section of its housing. As a compression load increases, the bearing seat comes into contact with the load-receiving section to limit the load absorbed by the elastic member. In this arrangement, the elastic member applies appropriate bearing pre-load to overcome manufacturing tolerances in the parts, without being required to support large compression loads.
A ball joint described in the above U.S. Patent has a problem with increased assembly time and costs with the need to fit and orient multiple parts into the inner chamber. Under compression loads for longer durations, the elastic member may be permanently deformed and lose its ability to compensate for dimensional tolerances when the compression load is lessened or removed. The materials used for the elastic member are likely to be more sensitive to damage by lubricants than the bearing seat material causing loss of resiliency with resulting loss in ability to compensate for the dimensional tolerances of the ball joint's parts as well as causing other damage to the ball joint.